1. Field of the Invention
The present invention relates generally to a planetary gear assembly. More particularly, the invention relates to a small backlash planetary gear assembly which has high torque capacity with reduced backlash.
2. Description of the Related Art
A small backlash planetary gear assembly is desirable so as to reduce a gap at a meshing portion of gears and at a bearing portion of a planet gear. It is difficult to maintain equal load distribution on respective planet gears and smooth rotation thereof. Also, a reduced gap may produce adverse effects when correcting for displacement of a sun gear in a direction parallel to a sun gear axis. Accordingly, maintaining equal load distribution to respective planet gears, correcting for angular transmission error, and correcting for displacement of a sun gear in the direction parallel to the sun gear axis are typical design considerations of the small backlash planetary gear assembly.
Small backlash planetary gear assemblies typically include internal gears having pitch circle diameters of approximately 60 mm. This size of gear exhibits small deflections when examining a relationship between a rated load and flexural spring rigidity. Therefore, it is difficult to absorb the influence of tolerance buildup in the gear machining and during assembly with respect to the typical characteristics set forth above and in view of the resiliency of respective elements.
In the small backlash planetary gear assembly, tolerance in gear machining and tolerance in assembling inherently influences various characteristics of the final products. A mechanism for adjusting fluctuation of backlash due to such tolerance in gear machining and tolerance in assembling has been proposed in commonly owned Japanese Unexamined Utility Model Publication No. Heisei 4-44545.
A backlash adjusting mechanism disclosed in the above-identified publication is designed to divide a carrier supporting four planet gears into halves along an axial direction for supporting respective two planet gears on divided carrier segments, and for relatively twisting respective carrier segments for adjusting the backlash between the gears.
With this backlash adjusting mechanism, two out of four planet gears may be meshed with the internal gear and the sun gear in a backlash free condition to successfully reduce lost motion.
However, in the backlash adjusting mechanism set forth above, only two out of four planet gears may contribute for torque transmission, thereby reducing torque capacity.